Composition and Origin of Jurassic Ammonite Concretions at Gerzen, Germany
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JURASSIC AMMONITE CONCRETIONS COMPOSITION AND ORIGIN OF JURASSIC AMMONITE CONCRETIONS AT GERZEN, GERMANY. By MICHAEL DAVID GERAGHTY, B.Sc. A Thesis Submitted to the School of Graduate Studies in Partial Fulfilment of the Requirements for the Degree Master of Science McMaster University (c) Copyright by Michael David Geraghty, April 1990 MASTER OF SCIENCE (1990) McMaster University (Geology) Hamilton, Ontario TITLE: Composition and Origin of Jurassic Ammonite Concretions at Gerzen, Germany. AUTHOR: Michael David Geraghty, B. Sc. (University of Guelph) SUPERVISOR: Professor G.E.G. Westermann NUMBER OF PAGES: xiii, 154, 17 Figs., 10 Pls. ii ACKNOWLEDGEMENTS I would like to express my sincere gratitude to Dr. Gerd Westermann for allowing me the privilege of studying under his supervision on a most interesting research project. His advice, support and patience were greatly appreciated. I deeply indebted to Mr. Klaus Banike of Gottingen, F. R. Germany for opening his home and his collection of concretions to me and also for his help and friendship. To Erhardt Trute and Family of Gerzen, F.R. Germany, I owe many thanks for their warm hospitality and assistance with my field work. Also Dr. Hans Jahnke of Georg-August University, Gottingen deserves thanks for his assistance and guidance. Jack Whorwood's photographic expertise was invaluable and Len Zwicker did an excellent job of preparing my thin sections. Also, Kathie Wright did a great job helping me prepare my figures. Lastly, I would like to thank all those people, they know who they are, from whom I begged and borrowed time, equipment and advice. iii ABSTRACT Study of the ecology of concretion and host sediment fossils from a shell bed in middle Bajocian clays of northwestern Germany indicates a predominantly epifaunal suspension-feeding community living on a firm mud bottom. The shell bed, firm bottom and low turbidity required by suspension feeders suggests a hiatus or reduced sedimentation at the time. Depth estimates of 50 to 150 m are indicated by ammonite and belemnite siphuncle and septal strength indices. Preservation of calcitic fossils is excellent in both concretions and host sediments. Aragonitic fossils show good preservation in the concretions. Aragonite has been replaced by at least two generations of calcite. Preservation in the host sediments is poor. Pyrite is common in void spaces of concretion fossils but less so in those from the host sediments. Eight concretions were studied, containing numerous Stephanoceras mutabile (macroconch) and Stephanoceras quenstedti (microconch) . These are of opposite sexes but are not a dimorphic pair. Distribution of shell debris and other fossils within the concretions suggests that the ammonites were swept by currents into shallow depressions in the sea floor lined with shell debris. Such depressions have been observed in modern iv sediments as the result of the feeding activity of rays. The presence of currents is indicated by the southwest orientation of belemnite rostra in the host sediments. Carbonate content of concretion matrix is high indicating concretion growth in very fluid muds. This contrast with the firm bottom indicated by fossil ecology suggests rapid burial of the benthic community by either a mudflow or a sudden, large increase in sedimentation. Concretion growth was initiated by decomposition of organic matter within the mud. v TABLE OF CONTENTS ACKNOWLEDGEMENTS . iii ABSTRACT . iv LIST OF TABLES X LIST OF PLATES xi LIST OF FIGURES . xii CHAPTER 1: INTRODUCTION 1 1.1.0 STATEMENT OF THE PROBLEM 1 1.2.0 CARBONATE CONCRETIONS 2 1.2.1 Relative Age .. 2 1.2.2 Trace Element and Mineral Geochemistry . 6 1.2.3 Stable Isotopes 8 1.2.3.1 Sulphur Isotopes 9 1.2.3.2 Oxygen Isotopes 11 1.2.3.3 Carbon Isotopes 12 1.2.4 Model for Concretion Formation 16 1.3.0 FOSSILS AND CONCRETIONS 21 1.3.1 Fossil-Concretion Associations 23 vi 1.3.2 Origin of Fossil Accumulations 25 1.3.3 Preservational bias and Paleoecology 30 1.3.4 Summary . 34 CHAPTER 2: MATERIALS AND METHODS 35 2.1.0 MATERIALS . 35 2.1.1 Stratigraphy 35 2.1.2 New Excavations 42 2.1.3 The Concretions and their Fauna 46 2.2.0 METHODS 54 2.2.1 Fossil Orientations 54 2.2.2 Cephalopod Shell Structure and Bathymetry . 55 2.2.3 Shell Dimensions and Measurements 59 2.2.4 Thin-sections 59 2.2.5 Carbonate Carbon 61 CHAPTER 3: TAPHONOMY 66 3.0 INTRODUCTION 66 3.1.0 PRESERVATION OF CALCITE SHELLS 67 3.2.0 PYRITIC INTERNAL MOLD 70 3.2.1 Source of the Pyrite 70 3.2.2 Pyrite in Fossils other than Ammonites 71 3.2.3 Pyrite in Ammonites 73 vii 3.3.0 SHELL REPLACEMENT 77 3.3.1 Ammonites 77 3.3.2 Bivalves and gastropods 79 3.3.3 Brachiopods 80 3.4.0 CLAY MOLDS . 81 3.5.0 CONCRETIONS 82 3.5.1 Carbonate Content 83 3.5.2 Ammonite Orientation . 84 3.5.3 Belemnite Orientation 88 CHAPTER 4: SYSTEMATIC DESCRIPTIONS AND HABITATS 94 4.1.0 CEPHALOPODS 94 4.2.0 HABITATS ... 106 4.2.1.0 Epifaunal Suspension Feeders 108 4.2.1.1 Bivalves .. 108 4.2.1.2 Brachiopods 111 4.2.1.3 Serpulid Worms 111 4.2.2.0 Infaunal Suspension Feeders 112 4.2.2.1 Shallow-infaunal Bivalves 112 4.2.2.2 Deep-infaunal Bivalves 113 4.2.3.0 Deposit Feeders 114 4.2.3.1 Bivalves . 114 4.2.3.2 Gastropods . 114 4.2.4.0 Carnivores and Scavengers 115 4.2.4.1 Echinoids 115 viii 4.2.4.2 Cephalopods 115 4.2.5 Ammonite and Belemnite Paleobathymetry . 116 4.3.0 PALEOCOMMUNITY RECONSTRUCTION 117 CHAPTER 5: DISCUSSION 129 5.1.0 PALEOECOLOGY 130 5.1.1 Fossil Accumulation 132 5.2.0 SUMMARY 138 REFERENCES 141 ix LIST OF TABLES Table 1.1: A comparison of species diversity between concretions and host shales in Pennsylvanian shales of the Dugger Formation . 32 Table 2.1: Correlation of ammonite beds (Schicten) and biozones for the Middle-Bajocian of northwestern Germany. 39 Table 3.1: Carbonate content (weight percent) of concretion matrix from concretions #301 and 313. 84 Table 4.1: Septal strength, siphuncle strength and septal flute strength indices for 4 Meqateuthis sp. and 4 Stephanoceras mutabile ¥ from Gerzen. 117 Table 4.2: Presumed habitats and diet of Gerzen invertebrate fauna at the concretion level 119 X LIST OF PLATES Plate 2.1: Concretions 63 Plate 2.2: Concretions 64 Plate 2.3: Concretions 65 Plate 3.1: Thin-sections 90 Plate 3.2: Concretion fauna 91 Plate 3.3: Thin-sections 92 Plate 3.4: Thin-sections . 93 Plate 4.1: Cephalopods 127 Plate 4.2: Host sediment fauna 128 Plate 5.1: Shell distribution in concretion 140 xi LIST OF FIGURES Figure 1.1: Rate of introduction and isotopic fractionation of diagenetic C02 • • • • 14 Figure 1.2: Schematic representation of the chemical processes producing local carbonate supersaturation at the site of a growing concretion . 20 Figure 1.3: Imbrication of small shells in erosional hollow behind larger shell . 29 Figure 2.1: Map of Northwestern Germany showing the location of Gerzen . 36 Figure 2.2: Map of the Gerzen claypit . 37 Figure 2.3: Correlation of composite profile of pits I and II and Westermann's (1954) main profile, at Gerzen. 45 Figure 2.4: Correlation of Septal Flute Strength versus Siphuncle Strength Index for the Ammonitina 58 Figure 2.5: Ammonite shell dimensions used in the systematic descriptions .. 60 Figure 3.1: Contoured stereonet diagrams showing orientation of the ammonites in concretions #309- 314 .. 86 Figure 3.2: Contoured stereonet diagrams showing orientation of the ammonites in concretions #315- xii 344 .. 87 Figure 3.3: Rose diagram of host sediment belemnite apex directions. 89 Figure 4.1: Graph of whorl expansion rate (Wr) versus diameter (D) for ~- mutabile ¥ and ~- guenstedti if. • . 97 Figure 4.2: Graph of whorl height (H) versus whorl width (W) for juvenile and adult whorls of ~- mutabile ¥ and ~- guenstedti if. 98 Figure 4.3: Graph of umbilical width (U) versus diameter (D) of ~- mutabile ¥ and ~- guenstedti if 99 Figure 4.5: Paleocommunity reconstruction of sea floor at Gerzen 121 Figure 5.1: Schematic diagram showing the distribution of shell debris . 135 Figure 5.2: Diagram showing how rays excavate feeding pits . 137 xiii CHAPTER 1: INTRODUCTION 1.1.0 STATEMENT OF THE PROBLEM In the early eighties, Mr. Klaus Banike, an amateur fossil collector from Gottingen, excavated several large carbonate concretions from an old abandoned claypit of Mid Jurassic muds, at Gerzen, near Alfeld, in northwestern Germany. The pit contains several horizons of concretions. The Banike concretions are exceptional for their abundant, well preserved ammonites . The ammonites are oriented from the horizontal to the vertical and include both macroconchs and microconchs of the genus Stephanoceras. Interestingly, these ammonites, but not the separately spaced concretions, were not reported by Westermann (1954) in his extensive study of the Gerzen claypit even though he sampled the horizon at which they were found. The purpose of this study is to determine, (l) the habitat of the fossils contained in the concretions and the local paleoenvironment at the time of deposition; (2) how the fossil accumulations contained in the concretions were formed; (3) the taphonomy of the fossils in the concretions, in particular the ammonites and why they were preserved in the l 2 concretions; ( 4) the species of the ammonite macro- and microconchs and whether or not they are a dimorphic pair. 1.2.0 CARBONATE CONCRETIONS Carbonate concretions are common in mudstone formations and are generally the most obvious diagenetic feature of these formations (Astin and Scotchman 1988) .